Less than 1% of the total body magnesium is present in the peripheral blood. We developed an assay to determine the magnesium content and concentration of blood mononuclear cells as an index for intracellular magnesium and/or total body magnesium status (plasma, red blood cell and mononuclear blood cell magnesium concentration) was assessed in normal individuals at six-month intervals and in patients with the premenstrual syndrome. The results show a seasonal variation in the mononuclear blood cell magnesium concentration.